Central Air Conditioning - Retrofitting Into an Old House

Retrofitting a central air conditioning system into an old building is certainly a challenge. There's always going to be some compromises as compared to systems designed and installed during new construction. There are mistakes, oversights and shortcuts however, that seem to plague A/C retrofits in old homes. These can end up being frustrating and costly issues for homeowners.

Bigger Is Not Necessarily Better

Installing the proper size system is very important for proper performance, efficiency, comfort and service life. An oversized system cycles on and off too frequently, resulting in reduced service life of the equipment. It can also leave too much moisture in the air because it cools too quickly. Even with the air dropping to the desired temperature, high humidity still feels uncomfortable. Undersized systems usually perform poorly and inefficiently. During extreme heat days or during heat waves, they can run constantly and never satisfy the thermostat.

The first step in an air conditioning installation should be many measurements of the building, including room sizes, window sizes and amount of insulation. The installer should size the equipment based on calculations and guidelines in the "Manual J" from the Air Conditioning Contractors of America.

Ducts and Air Flow: Where Science and Art Meet

Duct sizing and installation is part art and part science when retrofitting an old house. There's always going to be a compromise unless you happen to be gutting the entire home. Insufficient supply registers and undersized return air vents are typical. The biggest issue I find is installers leaving unsealed gaps and seams or inadequate duct support that results in seams separating. I often find filter slots without covers, allowing air into the system in the wrong location. Some of this air is bypassing the filter.

Condensation Drains

Air conditioning removes moisture from the air and the condensate that collects on the evaporator coil has to drain somewhere. I frequently find these drains directly connected to the nearest plumbing waste pipe or a vent pipe to waste system. People wondering why their house smells like a sewer when running the central air should check their drain termination.

When the air handler is located where leaking condensate could cause damage, like an attic, a pan with a drain should be installed, in addition to the primary drain. The drain pipe from the pan should be a complete separate piping system and terminate in a conspicuous location. More often than not, I've found the drain from the pan just connected to the primary drain.

Be Prepared for Collateral Damage

Installing a traditional, ducted air conditioning system is going to disrupt a home. Holes are going to be cut through floors, walls and ceilings. Vertical ducts are going to steal some closet space. Horizontal ducts are going to need to be concealed in soffits, built out where walls meet ceilings. What's disturbing is finding structural components have been hacked away, without any added reinforcement.

In a two-story home, with a basement and unfinished attic, some of the damage can be minimized by installing 2 separate systems. Supplying cold air to the first level from the basement and the second level from the attic eliminates the need for large ducts passing through the main living areas.

Other options include installing a high velocity system, that uses much smaller branch ducts or "split ductless" systems that make use of independent wall-mounted cooling units.

Be Sure to Read the Manual

The most important issue is to be sure the entire system is installed according to the manufacturer's instructions. These instructions often include national code requirements as well as important factors for optimum performance.

Unfortunately, I often come up against installers that don't think doing it by the book is important since "we've never done it that way and we been doin' this for decades."

About the AuthorWilliam Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company serving Southeastern Pennsylvania and Central New Jersey.